MicroRNAs (miRNAs) are small, noncoding RNAs that influence gene regulatory networks by post-transcriptional regulation of specific messenger RNA (mRNA) targets via specific base-pairing interactions. miRNAs have been shown to be present in human biofluids in a cell-free form. These cell-free miRNAs may be non-vesicular, bound and protected by proteins in miRNA-protein complexes, enclosed in membrane-bound vesicles such as exosomes or microvesicles, or both. Given the important functional role of miRNA in disease, this set of nucleic acid molecules contains candidates for diagnosing and prognosing disease, and monitoring response to therapies in a wide variety of patients and in subjects prior to manifesting disease in a readily available biological sample, such as blood serum and plasma, urine, or saliva. Current methods of isolating miRNAs are directed to relatively abundant miRNAs in cells and tissues, use spin columns which are not readily automated or scaled up, are complicated and involve toxic compounds, or may specifically isolate either vesicular or non-vesicular miRNAs. Furthermore, miRNAs of diagnostic or prognostic interest are often present at low abundance in biofluids, making their detection using current isolation methods challenging. Therefore, there is a need for a simple, efficient, automatable, and scalable method for isolating all or a majority of miRNAs in biofluids.